Use a Stereonet to Find the Plane Containing Two Lines

Example

Find the plane that contains two lines, one trending 214, plunging 40, and
the other
trending 128, plunging 50.

1. Plot the first line. In this case we have rotated the reference
(north) mark 34 degrees to the west and plotted the point (red) on the
north-south meridian (34 + 180 = 214). We count in 40 degrees from the primitive circle.

2. Plot the second line. Here we have rotated the reference (north)
mark 38 degrees to the west and plotted the point (green) on the equator (90 +
38 = 128). We count in 50 degrees from the primitive circle.

3. Rotate the overlay so the two points lie on a common great circle
(purple). Draw the great circle and count the dip in from the primitive circle.

4. Rotate the overlay to its original position and measure the strike.

This problem is virtually identical to the problem of finding
the angle between two lines. To measure the angle, we have to measure in the
plane that contains both lines. The only difference between the two problems is
whether we are more interested in the angle or in the plane.